Camera, image sensor, and method for decreasing undesirable dark current

ABSTRACT

A method for lowering dark current in an image sensor pixel, the method includes the steps of providing a photosensitive area for receiving incident light which is converted into a charge; providing a gate for transferring charge from the photosensitive area; wherein the gate is held at a voltage which will accumulate majority carriers at a semiconductor-dielectric interface during integration for the photosensitive area. Alternatively, a potentail profile can be provided under the gate to drain the dark current away from the photogeneration diffusion.

FIELD OF THE INVENTION

The invention relates generally to the field of image sensors and, moreparticularly, to such image sensors in which undesirable dark current issubstantially eliminated.

BACKGROUND OF THE INVENTION

As is well known in the art, dark current is a significant limitation ofthe performance of image sensors, particularly CMOS image sensors. Atypical image sensor includes a substrate having a photosensitive areaor charge collection area for collecting charge, and a transfer gate fortransferring charge from the photosensitive area to either acharge-to-voltage conversion mechanism, such as a floating diffusion ina CMOS image sensor, a transfer mechanism in a charge-coupled deviceimage sensor or to a reset mechanism. A dielectric is positioned betweenthe gate and the substrate, and the area of contact between the twoareas is generally referred to in the art as thesemiconductor/dielectric interface. During certain stages of imagecapture, such as integration, electrons not associated with thephotosensitive process that captures the electronic representation ofthe image, i.e., the photo-generation process, accumulate in certainportions of the sensor, such as adjacent gates, and inherently migrateinto the photosensitive area. These electrons, a portion of what iscalled dark current, are undesirable as they degrade the quality of thecaptured image.

It is known that a pinned photodiode includes substantially all theabove-described devices except as described hereinbelow. In this regard,pinned photodiodes include a photosensitive area with a pinned layerspanning the photosensitive area. Pinned photodiodes are known todecrease dark current in photosensitive areas. However, dark currentstill exists from adjacent gates.

Consequently, a need exists for substantially eliminating dark currentassociated with adjacent gates and other similar structures.

SUMMARY OF THE INVENTION

The present invention is directed to overcoming one or more of theproblems set forth above. Briefly summarized, according to one aspect ofthe present invention, the invention resides in a method for loweringdark current in an image sensor pixel, the method comprising the stepsof providing a photosensitive area for receiving incident light which isconverted into a charge; providing a gate for transferring charge fromthe photosensitive area; wherein the gate is held at a voltage whichwill accumulate majority carriers at a semiconductor-dielectricinterface during integration for the photosensitive area.

An alternative means to overcoming one or more of the problems set forthabove is presented where the potential profile under the adjoining gateis created so that dark current related to the gate is drained away fromthe photogeneration diffusion.

These and other aspects, objects, features and advantages of the presentinvention will be more clearly understood and appreciated from a reviewof the following detailed description of the preferred embodiments andappended claims, and by reference to the accompanying drawings.

ADVANTGEOUS EFFECTS OF THE INVENTION

The present invention has the advantage of substantially eliminatingdark current adjacent gates and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a typical art image sensor pixel;

FIG. 2 is a side view in cross section of FIG. 1;

FIG. 3 is a side view in cross section of the image sensor pixel of FIG.1 with a pinned-photodiode;

FIG. 4 is a side view in cross section of an image sensor pixel showingthe charge transfer channel from the photogeneration diffusion;

FIG. 5A is a potential profile along the charge transfer channel duringsignal integration in the photogeneration in the prior art;

FIG. 5B is a potential profile along the charge transfer channel duringcharge transfer from the photogeneration in the present invention;

FIG. 5C is a potential profile along the charge transfer channel duringsignal integration in the photogeneration with negative voltage;

FIG. 5D is a potential profile along the charge transfer channel duringsignal integration in the photogeneration in the present invention;

FIG. 5E is a potential profile along the charge transfer channel duringsignal integration in the photogeneration in an alternate embodiment ofthe present invention;

FIG. 6 is a top view of the image sensor including some on-chip andoff-chip circuitry; and

FIG. 7 is a camera for illustrating a typical commercial embodiment forthe image sensor of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1 and 2, there is shown the top and side view of apixel of an image sensor of the present invention. Although only onepixel 70 is shown, as is well known in the art, a plurality of suchpixels exists on an image sensor and only one is shown for clarity ofunderstanding. The image sensor includes a substrate 30, preferablysilicon, having a photosensitive area or charge collection area 20therein; the photogeneration takes place in the charge collection area20. The photosensitive area 20 receives incident light and consequentlyconverts the incident light into charge packets during imageintegration, as is well known in the art. The photosenstive area 20 iselectrically isolated from other areas of the pixel and other associatedcircuitry. A gate 10 having a dielectric 15 spanning its lower portionprovides a portion of this isolation, and the gate 10 can beelectrically biased to isolate the photosensitive area 20 or to permitthe charge collected in the photosensitive area 20 to flow into anadjoining charge-to-voltage conversion node 22 (or referred toalternatively as diffusion or charge sensing node) for purposes ofmeasurement of charge or resetting the charge collection area 20. Thegate-controlled charge transfer is along a path 50 that is formed bycreating a trough of potential minimum.

Undesirable dark current is generated both in the photosensitive area 20and along the charge transfer channel 50. Typically, a high rate of darkcurrent generation occurs both at the semiconductor/dielectric interface42 adjacent to the photosensitive area 20 and at thesemiconductor/dielectric interface 40 under the gate 10 due to the highrate of generation resulting from interface states. The dark currentfrom the interfaces 40 and 42 is the dominant source of dark currentflowing into the charge sensing node 22. It is noted that the chargesensing node 22 may be replaced by a reset node resulting in the samebehavior. For purposes of brevity in the present invention, theimplementation with the charge sensing node 22 will be discussed.

Referring to FIG. 3, there is shown the side view of a pixel where aheavily doped diffusion 32 opposite in type to that in the chargecollection area 20 is used to shield the charge collection area 20 fromthe interface 42. This is generally referred to in the art as a pinnedphotodiode pixel. The photogeneration and charge transfer is along path50 as before. The diffusion 32, among other benefits, has the effect tosuppress dark current generation at the semiconductor/dielectricinterface 42 adjacent to the charge collection area 20.

In this configuration, a dominant source of dark current is at thesemiconductor/dielectric interface or surface 40 under the gate 10. Thepresent invention presents a means of surpressing this dark current bybiasing the gate 10 to a potential so that the semiconductor at theinterface 40 becomes accumulated with free carriers of the majoritydoping type. The dark current generation occurs because the defects arein an non-equilibrium state, and this accumulation supresses thisgeneration by returning the region where the highest quantity of defectsoccur to local equilibrium.

Referring to FIG. 4, there is shown a side view in cross section of theimage sensor of the present invention as in FIG. 2. In the prior art,the interface 40 is biased in a non-equilibrium state resulting in thegeneration of dark current. The charge generated by photogeneration(desirable charge generated by the incident light for capturing theimage) and by dark current (undesirable charge generated by other meanswell known in the art) is collected within the charge collection area 20at a potential extremum 52. This signal charge is isolated duringintegration by a barrier created either at the charge-to-transferpotential transition 54 located between the potential extremum 52 andthe gate-associated charge transfer channel 56, or at thegate-associated charge transfer channel 56. The existence of either ofthese barriers is a result of the doping in the semiconductor 30 and thebias on the gate 10.

Referring to FIG. 5A, there is in the prior art the potential in thegate-associated channel or gate channel potential 56 where the potentialon gate forms a barrier that isolates the collection potential 52 fromthe destination potential 58 and where dark current charge can flow boththrough the collection-to-transfer potential transition 54 to add todark current in the collection potential 52 that adds to the signalcharge, and along the charge transfer path 50 to the destinationpotential 58. The result is that some of the dark current generated inthe interface under the gate 40 will contribute to the dark current inthe signal charge located at the destination potential 58.

Referring to FIG. 5B, there is shown that the potential profile of thepresent invention where the potential on gate 10 removes a barrier thatisolated the collection potential 52 from the destination potential 58and whereby signal charge is readout or reset from the charge collectionarea 20 along the path 50 to the destination potential 58. Before thisis accomplished and while the barrier is still present, however, thecharge in the destination potential 58 is removed by means commonlyknown in the art so that any dark current collected here is keptseparate from signal charge.

Referring to FIG. 5C, in a manner in the present invention, a mechanismis disclosed where the gate 10 is used to adjust the potential in thegate-associated channel 56, for example by applying a negative voltage,to the point where the potential on gate 10 forms a barrier thatisolates the collection potential 52 from the destination potential 58and whereby the semiconductor interface 40 is held in an equilibriumcondition. The equilibrium condition suppresses dark current from thisinterface 40 so that it does not contribute to signal dark currentcollected in the collection volume 52 and does not eventually betransferred along the channel to the destination potential 58.

In the present invention, an additional mechanism, in addition to theabove described biasing, is disclosed to eliminate the contribution ofdark current from the interface under the gate 10 and the chargetransfer channel under the gate 56. Referring to FIG. 5D, there is shownthat this dark charge can be directed toward the destination potential58 if a potential barrier to charge flow is formed at thecollection-to-transfer potential transition 54. The dark charge willflow to destination potential 58 where it can be removed before the gatebias is changed to transfer the signal charge (for example the imagesignal) along the potential path 50 to the destination potential 58 oris otherwise read out. Therefore, this dark current is kept separatefrom the signal charge collected at the destination potential 58. Such abarrier can be created as a result of the doping in the semiconductor 30and the bias on the gate 10.

Referring to FIG. 5E, as an additional embodiment of the presentinvention, the same result can be achieved if a potential gradient isformed along the transfer channel potential 56 causing the dark currentcharge generated at the interface under the gate 10 to preferentiallyflow to the destination potential 58 during the signal integration.Therefore, this dark current is kept separate from the signal charge orimage signal collected at the destination potential 58. Such a barriercan be created as a result of the doping in the semiconductor 30 thebias on the gate 10 and the bias on the destination potential 58.

Referring to FIG. 6, there is shown a top view of an image sensor 75having a plurality of pixels 70 and additional, on-chip circuitry orgeneration source 80 which includes circuitry that enables operation ofthe above-described, more specifically biasing of the gates 10.Alternatively, this circuitry may be implemented by off-chip or externalcircuitry 90.

Referring to FIG. 7, there is shown a camera 200 that includes the imagesensor 75 of the present invention for illustrating a typical commercialembodiment.

The invention has been described with reference to preferredembodiments. However, it will be appreciated that variations andmodifications can be effected by a person of ordinary skill in the artwithout departing from the scope of the invention.

PARTS LIST

-   10 gate-   15 dielectric-   20 photosensitive or charge collection area-   22 charge-to-voltage conversion node (or referred to alternatively    as diffusion or charge sensing node)-   30 substrate/semiconductor-   32 heavily doped diffusion-   40 semiconductor/dielectric interface-   42 semiconductor/dielectric interface-   50 path or gate controlled charge transfer channel-   52 potential extremium or collection potential-   54 charge-to-transfer potential transition or collection-to-transfer    potential transition-   56 gate-associated charge transfer channel or gate channel potential-   58 destination potential-   70 pixels-   75 image sensor-   80 on-chip circuitry or generation source-   90 off-chip or external circuitry-   200 camera

1. A method for lowering dark current in an image sensor pixel, themethod comprising the steps of: (a) providing a photosensitive area forreceiving incident light which is converted into a charge; and (b)providing a gate for transferring charge from the photosensitive area;wherein the gate is held at a voltage which will accumulate majoritycarriers at a semiconductor-dielectric interface associated with thegate during integration for the photosensitive area.
 2. The method as aclaim 1 for comprising the step of providing a charge-to-voltageconversion node that receives the charge from the photosensitive area.3. The method as a claim 1, wherein step (b) comprises providing anon-chip generation source for generating the voltage.
 4. The method as aclaim 1, wherein step (b) comprises providing an external voltage sourcefor generating the voltage.
 5. The method as a claim 1 furthercomprising the step of providing a gate channel potential so that chargegenerated at a surface adjacent the gate is directed away from thephotosensitive area.
 6. The method as a claim 1 for comprising the stepof providing a negative voltage as the voltage that will accumulatecarriers at a semiconductor-dielectric interface.
 7. The step as a claim1 further comprising the step of providing silicon as the semiconductorand silicon dioxide as the dielectric.
 8. A method for lowering darkcurrent in an image sensor pixel, the method comprising the steps of:(a) providing a substrate; (b) providing a photosensitive area in thesubstrate for receiving incident light that is converted into a charge;and (c) providing a gate for transferring charge from the photosensitivearea; wherein step (a) includes providing the substrate substantiallyadjacent the gate with an impurity concentration that transfers darkcurrent away from the photosensitive area and into a diffusion on anopposite side from the photosensitive area during integration for thephotosensitive area.
 9. An image sensor pixel that substantiallyeliminates dark current, the sensor comprising: (a) a photosensitivearea for receiving incident light that is converted into a charge; and(b) a gate for transferring charge from the photosensitive area; whereinthe gate, when at a predetermined voltage, will accumulate majoritycarriers substantially at a semiconductor-dielectric interfaceassociated with the gate during integration for the photosensitive area.10. The image sensor as in claim 9 further comprising acharge-to-voltage conversion node that receives the charge from thephotosensitive area.
 11. The image sensor as in claim 9, furthercomprising an on-chip generation source for generating the voltage asthe predetermined voltage.
 12. The image sensor as in claim 9 furthercomprising an external voltage source for generating the voltage. 13.The image sensor as in claim 9, wherein the predetermined voltage is anegative voltage that will accumulate carriers at asemiconductor-dielectric interface.
 14. The image sensor as in claim 9,wherein silicon is provided as the semiconductor and silicon dioxide isprovided as the dielectric.
 15. An image sensor pixel comprising: (a) asubstrate; (b) a photosensitive area in the substrate for receivingincident light that is converted into a charge; and (c) a gate fortransferring charge from the photosensitive area; wherein the substratesubstantially adjacent the gate includes an impurity concentration thattransfers dark current away from the photosensitive area and into adiffusion on an opposite side from the photosensitive area duringintegration for the photosensitive area.
 16. A camera comprising: animage sensor pixel that substantially eliminates dark current, thesensor comprising: (a) a photosensitive area for receiving incidentlight that is converted into a charge; and (b) a gate for transferringcharge from the photosensitive area; wherein the gate, when at apredetermined voltage, will accumulate majority carriers substantiallyat a semiconductor-dielectric interface during integration for thephotosensitive area.
 17. The camera as in claim 16 further comprising acharge-to-voltage conversion node that receives the charge from thephotosensitive area.
 18. The camera as in claim 16, further comprisingan on-chip generation source for generating the voltage as thepredetermined voltage.
 19. The camera as in claim 16 further comprisingan external voltage source for generating the voltage.
 20. The camera asin claim 16, wherein the predetermined voltage is a negative voltagethat will accumulate carriers at a semiconductor-dielectric interface.21. The camera as in claim 16, wherein silicon is provided as thesemiconductor and silicon dioxide is provided as the dielectric.
 22. Animage sensor pixel comprising: (a) a substrate; (b) a photosensitivearea in the substrate for receiving incident light that is convertedinto a charge; and (c) a gate for transferring charge from thephotosensitive area; wherein the substrate substantially adjacent thegate includes an impurity concentration that transfers dark current awayfrom the photosensitive area and into a diffusion on an opposite sidefrom the photosensitive area during integration for the photosensitivearea.